Light emitting device

ABSTRACT

A triplet light emitting device which has high efficiency and improved stability and which can be fabricated by a simpler process is provided by simplifying the device structure and avoiding use of an unstable material. In a multilayer device structure using no hole blocking layer conventionally used in a triplet light emitting device, that is, a device structure in which on a substrate, there are formed an anode, a hole transporting layer constituted by a hole transporting material, an electron transporting and light emitting layer constituted by an electron transporting material and a dopant capable of triplet light emission, and a cathode, which are laminated in the stated order, the combination of the hole transporting material and the electron transporting material and the combination of the electron transporting material and the dopant material are optimized.

The instant Application is Continuation of prior application Ser. No.10/737,569 filed Dec. 16, 2003 (now allowed), which is a continuation ofapplication of application Ser. No. 10/304,410 filed Nov. 26, 2002, nowU.S. Pat. No. 6,734,457.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic light emitting deviceconstituted by an anode, an organic compound film capable of emittinglight under the action of an electric field, and a cathode. Inparticular, the present invention relates to an organic light emittingdevice using a light emitting material which emits light in a tripletexited state.

2. Description of the Related Art

An organic light emitting device is a device designed by utilizing aphenomenon in which electrons and holes are caused to flow into anorganic compound film through two electrodes by application of a voltageto cause emission of light from molecules in an excited state (excitedmolecules) formed by recombination of the electrons and holes.

Emission of light from an organic compound is a conversion into light ofenergy released when excited molecules are formed and then deactivatedinto the ground state. Deactivation processes causing such emission oflight are broadly divided into two kinds: a process in whichdeactivation proceeds via a state in which excited molecules are singletexcited molecules (in which fluorescence is caused), and a process inwhich excited molecules are triplet excited molecules. Deactivationprocesses via the triplet excited molecule state include an emissionprocess in which phosphorescence is caused and a triplet-tripletextinction process. However, there are basically only a small number oforganic materials capable of changing in accordance with thephosphorescent deactivation process at room temperature. (In ordinarycases, thermal deactivation different from deactivation with emission oflight occurs.) The majority of organic compounds used in organic lightemitting devices are materials which emit light by fluorescence via thesinglet excited molecule state, and many organic light emitting devicesuse fluorescence.

Organic light emitting devices using such organic compounds capable ofemitting light by fluorescence are based on the two-layer structurewhich was reported by C. W. Tang et al. in 1987 (Reference 1: C. W. Tangand S. A. Vanslyke, “Organic electroluminescent diodes”, Applied PhysicsLetters, Vol. 51, No. 12, 913-915 (1987)), and in which an organiccompound film formed of layers of two or more organic compounds andhaving a total thickness of about 100 nm is interposed betweenelectrodes. Adachi et al. thereafter proposed a three-layer structure in1988 (Reference 2: Chihaya ADACHI, Shozuo TOKITO, Tetsuo TSUTSUI andShogo SAITO, “Electroluminescence in Organic Films with Three-LayeredStructure”, Japanese Journal of Applied Physics, Vol. 27, No. 2,L269-L271 (1988)). Multilayer device structures based on applications ofthese layered structures are being presently used.

Devices in such multilayer structures are characterized by “layerfunction separation”, which refers to the method of separately assigningfunctions to layers, instead of making one organic compound have variousfunctions. For example, a device of two-layer structure uses a holetransporting layer having the function of transporting positive holes,and a light emitting and electron transporting layer having the functionof transporting electrons and the function of emitting light. Also, adevice of three-layer structure uses a hole transporting layer havingonly the function of transporting positive holes, an electrontransporting layer having only the function of transporting electrons,and a light-emitting layer which is capable of emitting light, and whichis formed between the two transporting layers. Such a layer functionseparation method has the advantage of increasing the degree ofmolecular design freedom of organic compounds used in an organic lightemitting device.

For example, a number of characteristics, such as improved facility withwhich either of electrons and holes are injected, the function oftransporting both the carriers, and high fluorescent quantum yield, arerequired of a device of single-layer structure. In contrast, in the caseof a device of two-layer structure or the like using an electrontransporting and light emitting layer, an organic compound to whichpositive holes can be easily injected may be used as a material for ahole transporting layer, and an organic compound to which electrons canbe easily injected and which have high fluorescent quantum yield may beused as a material for an electron transporting layer. Thus,requirements of one layer are reduced and the facility with which thematerial is selected is improved.

In the case of a device of three-layer structure, a “light emittinglayer” is further provided to enable separation between the electrontransporting function and the light emitting function. Moreover, amaterial in which a fluorescent pigment (guest) of high quantum yieldsuch as a laser pigment is dispersed in a solid medium (host) materialcan be used for the light emitting layer to improve the fluorescentquantum yield of the light emitting layer. Thus, not only the effect oflargely improving the quantum yield of the organic light emitting devicebut also the effect of freely controlling the emission wavelengththrough the selection of fluorescent pigments to be used can be obtained(Reference 3: C. W. Tang, S. A. Vanslyke and C. H. Chen,“Electroluminescence of doped organic thin films”, Journal of AppliedPhysics, Vol. 65, 3610-3616 (1989)). A device in which such a pigment(guest) is dispersed in a host material is called a doped device.

Another advantage of the multilayer structure is a “carrier confinementeffect”. For example, in the case of the two-layer, structure describedin Reference 1, positive holes are injected from the anode into the holetransporting layer, electrons are injected from the cathode into theelectron transporting layer, and the holes and electrons move toward theinterface between the hole transporting layer and the electrontransporting layer. Thereafter, while holes are injected into theelectron transporting layer because of a small ionization potentialdifference between the hole transporting layer and the electrontransporting layer, electrons are blocked by the hole transporting layerto be confined in the electron transporting layer without being injectedinto the hole transporting layer because the electrical affinity of thehole transporting layer is low and because the difference between theelectrical affinities of the hole transporting layer and the electrontransporting layer is excessively large. Consequently, both the densityof holes and the density of electrons in the electron transporting layerare increased to achieve efficient carrier recombination.

As an example of a material that is effective in exhibiting the carrierconfinement effect, there is a material having an extremely largeionization potential. It is difficult to inject holes into the materialhaving a large ionization potential, so that such a material is widelyused as a material capable of blocking holes (hole blocking material).For example, in the case where the hole transporting layer composed ofan aromatic diamine compound and the electron transporting layercomposed of tris(8-quinolinolato)-aluminum (hereinafter referred to as“Alq”) are laminated as reported in Reference 1, if a voltage is appliedto the device, Alq in the electron transporting layer emits light.However, by inserting the hole blocking material between the two layersof the device, holes are confined in the hole transporting layer, sothat light can be emitted from the hole transporting layer side as well.

As described above, layers having various functions (hole transportinglayer, hole blocking layer, electron transporting layer, electroninjection layer, etc.) are provided to improve the efficiency and toenable control of the color of emitted light, etc. Thus, multilayerstructures have been established as the basic structure for currentorganic light emitting devices.

Under the above-described circumstances, in 1998, S. R. Forrest et al.made public a doped device in which a triplet light emitting materialcapable of emission of light (phosphorescence) from a triplet excitedstate at a room temperature (a metal complex having platinum as acentral metal in the example described in the reference) is used as aguest (hereinafter referred to as “triplet light emitting device)(Reference 4: M. A. Baldo, D. F. O'Brien, Y You, A. Shoustilkov, S.Silbley, M. A. Thomoson and S. R. Forrest, “Highly efficientphosphorescent emission from organic electroluminescent devices”,Nature, Vol. 395, 151-154 (1998)). For distinction between this tripletlight emitting device and devices using emission of light from a singletexcited state, the latter device will be referred to as “singlet lightemitting device”.

As mentioned above, excited molecules produced by recombination of holesand electrons injected into an organic compound include singlet excitedmolecules and triplet excited molecules. In such a case, singlet excitedmolecules and triplet excited molecules are produced in proportions of1:3 due to the difference between the multiplicities of spin. Basically,in the existing materials, triplet excited molecules are thermallydeactivated at room temperature. Therefore only singlet excitedmolecules have been used for emission of light, only a quarter ofproduced excited molecules are used for emission of light. If tripletexcited molecules can be used for emission of light, a light emissionefficiency three to four times higher than that presently achieved canbe obtained:

In Reference 4, the above-described multilayer structure is used. Thatis, the device is such structured that:4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereinafter referredto as “α-NPD”) that is an aromatic amine-based compound, is used as thehole transporting layer; Alq with 6% of2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin-platinum (hereinafterreferred to as “PtOEP”) dispersed therein is used as the light emittinglayer; and Alq is used as the electron transporting layer. As to theexternal quantum efficiency, the maximum value is 4% and a value of 1.3%is obtained at 100 cd/m².

Thereafter, the device structure utilizing the hole blocking layer isused. That is, the device is such structured that: α-NPD is used as thehole transporting layer; 4,4′-N,N′-dicarbazole-biphenyl (hereinafterreferred to as “CBP”) with 6% of PtOEP dispersed therein is used as thelight emitting layer; 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline(hereinafter referred to as “BCP”) is used as the hole blocking layer;and Alq is used as the electron transporting layer. As to the externalquantum efficiency, a value of 2.2% is obtained at 100 cd/m² and themaximum value is 5.6%, so that the light emission efficiency of thedevice is improved (Reference 5: D. F O'Brien, M. A. Baldo, M. E.Thompson and S. R. Forrest, “Improved energy transfer inelectrophosphorescent devices”, Applied Physics Letters, Vol. 74, No. 3,442-444 (1999)).

Further, a triplet light emitting device is reported which usestris(2-phenylpyridine)iridium (hereinafter referred to as “Ir(ppy)₃”) asthe triplet light emitting material (Reference 6: M. A. Baldo, S.Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, “Veryhigh-efficiency green organic light-emitting devices based onelectrophosphorescence”, Applied Physics Letters, Vol. 75, No. 1, 4-6(1999)). Thereafter, it is reported that with the same device structureas in Reference 6, the film thicknesses of the organic compound filmsare optimized, whereby a highly efficient organic light emitting deviceis obtained whose external quantum efficiency is 14.9% at 100 cd/m²(Reference 7: Teruichi Watanabe, Kenji Nakamura, Shin Kawami, YoshinoriFukuda, Taishi Tsuji, Takeo Wakimoto, Satoshi Miyaguchi, MasayukiYahiro, Moon-Jae Yang, Tetsuo Tsutsui, “Optimization of emittingefficiency in organic LED cells using Ir complex”, Synthetic Metals,Vol. 122, 203-207 (2001)). Thus, in actuality, it becomes possible toproduce the devices with the light emission efficiency almost threetimes that in the conventional singlet light emitting device.

Searches are presently being made for triplet light emitting materialsusing iridium or platinum as a central metal, triplet light emittingdevices having markedly high efficiency in comparison with singlet lightemitting devices are now attracting attention, and studies about suchdevices are being energetically made.

Although triplet light emitting devices have light emission efficiencymuch higher than that of singlet light emitting devices, they areincomparably shorter in life than singlet light emitting materials andlack stability. Also, a multilayer structure adopted to increase theefficiency of a triplet light emitting device must be formed so as tohave at least four layers. Therefore triplet light emitting devicessimply have the drawback of requiring much time and labor forfabrication.

With respect to the life of devices, a report has been made which saysthat the half-life of a device having a multilayer structure formed of ahole transporting layer using α-NPD, a light emitting layer using CBP asa host material and Ir(ppy)₃ as a guest (dopant) material, a holeblocking layer using BCP, and an electron transporting layer using Alqis only 170 hours under a condition of an initial luminance of 500 cd/m²(Reference 8: Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, KenjiNAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, TakeoWAKIMOTO and Satoshi MIYAGUCHI, “High Quantum Efficiency InorganicLight-Emitting Devices with Iridium-Complex as a Triplet EmissiveCenter”, Japanese Journal of Applied Physics, Vol. 38, No. 12B,L1502-L1504 (1999)). By considering this life, it must be said that nosolution of the life problem is close at hand.

In Reference 8, low stability of BCP used as a hole blocking material ismentioned as a cause of the limitation of life. Triplet light emittingdevices use as a basic structure the device structure described inReference 5, and use the hole blocking layer as an indispensableelement. FIG. 12 show the structure of a conventional triplet lightemitting device. In the device structure shown in FIG. 12, an anode 1102is formed on a substrate 1101, a multilayer organic compound film formedof a hole transporting layer 1103, a light emitting layer 1104, a holeblocking layer 1105, and an electron transporting layer 1106 is formedon the anode 1102, and a cathode 1107 is formed on the multilayer film.While efficient carrier recombination can be achieved by the carrierconfinement effect of the hole blocking layer, the life of the device islimited because the hole blocking material ordinarily used isconsiderably low in stability. Also, CBP used as a host material is alsolow in stability and is also considered to be a cause of the limitationof the life.

A device of three-layer structure using no hole blocking layer has beenfabricated (Reference 9: Chihaya ADACHI, Marc A. Baldo, Stephen R.Forrest and Mark E. Thompson, “High-efficiency organicelectrophosphorescent devices with tris(2-phinylpyridine) iridium dopedinto electron-transporting materials”, Applied Physics Letters, Vol. 77,No. 6, 904-906 (2000)). This device is characterized by using electrontransporting materials as a host material instead of CBP which is the tohave such characteristics as to transport both the carriers. However,the electron transporting materials used as a host material are BCPwhich is used as a hole blocking material,1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxazole (hereinafter referred to as“OXD7”), and 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole(hereinafter referred to as “TAZ”). Although no hole blocking layer isused, the materials ordinarily used as a hole blocking material are usedin the device. BCP is, of course, lower in stability than any othermaterial, so that the stability of the device is low, while theefficiency is high.

A simple two-layer device structure using no hole blocking material hasalso been reported (Reference 10: Chihaya ADACHI, Raymond KWONG, StephenR. Forrest, “Efficient electrophosphorescence using a doped ambipolarconductive molecular organic thin film”, Organic Electronics, Vol. 2,37-43 (2001)). In this device, however, CBP is used as a host material,so that the stability is low, while the light emission efficiency ishigh.

As described above, while triplet light emitting devices having highlight emission efficiency have been reported, no triplet light emittingdevice improved both in efficiency and in stability has been reported.Difficulty in obtaining such an improved device is due to theinstability of host materials and hole blocking materials used.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a triplet lightemitting device in which unstable materials such as those describedabove are not used while the device structure is simplified to obtainhigh efficiency and improved stability, and which can be fabricatedeasily and efficiently in comparison with the conventional devices.

According to the present invention, a triplet light emitting devicedesigned to achieve the above-described object has a simple devicestructure (FIG. 1) in which no hole blocking layer such as that providedin the conventional triplet light emitting devices is used, and in whichan organic compound film is formed as a multilayer film constituted by ahole transporting layer and a layer in which dopant material capable oftriplet light emission is dispersed in a stable electron transportingmaterial. That is, a device structure is provided in which an anode 102is formed on a substrate 101, a hole transporting layer 103 constitutedby a hole transporting material, an electron transporting and lightemitting layer 104 constituted by an electron transporting material anda dopant material capable of triplet light emission are successivelyformed on the anode 102, and a cathode 105 is formed on the layer 104.The region interposed between the anode 102 and the cathode 105 (i.e.,the hole transporting layer 103 and the electron transporting and lightemitting layer 104) corresponds to the organic compound film.

The present invention is characterized in that, in an organic lightemitting device constituted by an anode, an organic compound film, and acathode, the organic compound film includes a hole transporting layerconstituted by a hole transporting material, and an electrontransporting layer formed adjacent to the hole transporting layer andconstituted by an electron transporting material, and a light emittingmaterial capable of emitting light from a triplet excited state is addedin the electron transporting layer.

A hole injection layer may be inserted between the anode 102 and thehole transporting layer 103. Also, an electron injection layer may beinserted between the cathode 105 and the electron transporting and lightemitting layer 104. Further, both the hole injection layer and theelectron injection layer may be inserted.

As a means for achieving the object of the present invention, it isimportant to consider the combination of a hole transporting materialand an electron transporting material in the above-described device forpreventing emission of light from the hole transporting layer 103.

Accordingly, the present invention is characterized in that the energydifference between the highest occupied molecular orbit level and thelowest unoccupied molecular orbit level in the hole transportingmaterial is larger than the energy difference between the highestoccupied molecular orbit level and the lowest unoccupied molecular orbitlevel in the electron transporting material.

Another means for achieving the object resides in avoiding overlapbetween an absorption spectrum of the hole transporting material and alight emission spectrum of the electron transporting material. In thiscase, it is preferred not only that the spectrums do not overlap eachother, but also that the positional relationship between the spectrumsbe such that the absorption spectrum of the hole transporting materialis on the shorter-wavelength side of the light emission spectrum of theelectron transporting material.

As a means for achieving the object of the present invention, it isimportant to adopt a device arrangement enabling the dopant capable oftriplet light emission to easily trap the carriers in improving thelight emission efficiency of the above-described triplet light emittingdevice of the present invention.

Accordingly, the present invention is characterized in that both thehighest occupied molecular orbit level and the lowest unoccupiedmolecular orbit level of the light emitting material capable of emittinglight from a triplet excited state are in the energy gap between thehighest occupied molecular orbit level and the lowest unoccupiedmolecular orbit level of the electron transporting material.

As still another means for achieving the object of the presentinvention, the light emitting device is characterized in that the valueof ionization potential of the hole transporting material is equal to orlarger than the value of ionization potential of the light emittingmaterial capable of emitting light from a triplet excited state.

Further, as another means for achieving the object of the presentinvention, the light emitting device is characterized in that theabsolute value of a value indicating the lowest unoccupied molecularorbit level of the hole transporting material is smaller by 0.2 eV ormore than the absolute value of a value indicating the lowest unoccupiedmolecular orbit level of the electron transporting material.

It is more preferable to use a device arrangement corresponding to acombination of these means, i.e., an arrangement in which the value ofionization potential of the hole transporting material is equal to orlarger than the value of ionization potential of the light emittingmaterial capable of emitting light from a triplet excited state, and theabsolute value of a value indicating the lowest unoccupied molecularorbit level of the hole transporting material is smaller by 0.2 eV ormore than the absolute value of a value indicating the lowest unoccupiedmolecular orbit level of the electron transporting material.

In view of the above description, the present invention is characterizedin that used as the preferred hole transporting material is one selectedfrom the group consisting of 4,4′,4″-tris(N-carbazole)triphenylamine,4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane,1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene,1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene; and1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene.

Further, the present invention is characterized in that used as theelectron transporting material is one selected from the group consistingof 2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzoimidazole],lithiumtetra(2-(2-hydroxyphenyl)benzooxazolatoboron,bis(2-(2-hydroxyphenyl)benzooxazolato)(triphenylsilanolato)aluminum,bis(2-(2-hydroxyphenyl)benzothiazolato)(triphenylsilanolato)aluminum,2-(2-hydroxyphenyl)benzooxazolatolithium,(2-(2-hydroxyphenyl)benzooxazolato)-diphenylboron,tris(8-quinolinolato)-aluminum,bis(2-methyl-8-quinolinolato)(triphenylsilanolato)aluminum,bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum,lithiumtetra(2-methyl-8-hydroxy-quiriolinato)boron,(2-methyl-8-quinolinolato)-diphenylboron, andbis(2-methyl-8-quinolinolato)aluminiumhydroxide.

Further, in the device of the present invention, it is effective to usethe hole transporting material and the electron transporting material incombination.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing the device structure of a two-layer tripletlight emitting device in accordance with the present invention;

FIG. 2 is a diagram showing HOMO and LUMO energy levels;

FIG. 3 is an energy gap diagram of the device;

FIGS. 4A and 4B are diagrams showing the positional relationship betweenthe light emission spectrum of a host material and the absorptionspectrum of a hole transporting material;

FIGS. 5A to 5D are graphs showing an initial characteristic and a lightemission spectrum in Embodiment 1;

FIGS. 6A to 6D are graphs showing an initial characteristic and a lightemission spectrum in Embodiment 2;

FIGS. 7A to 7D are graphs showing an initial characteristic and a lightemission spectrum in Embodiment 3;

FIGS. 8A to 8D are graphs showing an initial characteristic and a lightemission spectrum in Embodiment 4;

FIGS. 9A to 9D are graphs showing an initial characteristic and a lightemission spectrum in Comparative Example 1;

FIGS. 10A to 10D are graphs showing an initial characteristic and alight emission spectrum in Comparative Example 2;

FIGS. 11A to 11D are graphs showing an initial characteristic and alight emission spectrum in Comparative Example 3; and

FIG. 12 is a diagram showing the device structure of a conventionaltriplet light emitting device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment mode of the present invention will be described in detail.An organic light emitting device may have at least one of an anode and acathode made transparent to enable emitted light to be output. However,the embodiment mode of the present invention will be described withrespect to a device structure in which a transparent anode is formed ona substrate and light is output through the anode. In actuality, thepresent invention can also be applied to a structure in which a cathodeis formed on a substrate and light is output through the cathode, astructure in which light is output from the side opposite from asubstrate, and a structure in which light is output through opposedelectrodes.

As described above, the present invention is characterized in that useof a hole blocking layer in a triplet light emitting device is avoided(FIG. 1). However, the present invention is different from a method offabricating a device designed only by removing the hole blocking layerfrom the conventional device structure (FIG. 12).

The conventional triplet light emitting device and the two-layer deviceof the present invention have different recombination regions. In theconventional triplet light emitting device, a hole blocking layer isused and the carrier recombination region corresponds to the interfacebetween the light emitting layer and the hole blocking layer. Incontrast, in the device structure proposed in the present invention, thecarrier recombination region corresponds to the interface between thehole transporting layer and the electron transporting material providedas a host material.

Therefore it is important to consider a light emission mechanism intriplet light emitting devices. In general, two kinds of light emissionmechanisms are conceivable as light emission mechanisms in devices usinga guest-host light emitting layer using a dopant (guest).

The first light emission mechanism is emission from the dopant caused bytransfer of energy from the host. In this case, both the carriers areinjected into the host to form excited molecules of the host. The energyof the excited molecules is transferred to the dopant. The dopant isexcited by the energy and emits light when deactivated. In triplet lightemitting devices, the dopant is a material for emitting phosphorescencevia a triplet excited molecule state, and light is therefore emitted byphosphorescence.

In the light emission mechanism based on transfer of energy, themagnitude of overlap of the light emission spectrum of the host materialand the absorption spectrum of the dopant material is important. Thepositional relationship between the highest occupied molecular orbit(HOMO) and the lowest unoccupied molecular orbit (LUMO) in the hostmaterial and the dopant material is not important.

In this specification, the value of ionization potential measured byphotoelectron spectrometry in atmospheric air is used as the value ofthe HOMO. The absorption ends of the absorption spectrum define theenergy difference between the HOMO and the LUMO (hereinafter referred toas “energy gap value”). Therefore the value obtained by subtracting theenergy gap value estimated from the absorption ends of the absorptionspectrum from the value of ionization potential measured byphotoelectron spectrometry is used as the value of the LUMO. Inactuality, these values (HOMO (ionization potential), LUMO (energy gapvalue)) are negative since they are measured with reference to thevacuum level. However, they are shown as absolute values throughout thespecification. Conceptual views of the HOMO, the LUMO, and the energygap values are as shown in FIG. 2.

If both the energy levels of the HOMO and LUMO of the dopant materialare placed in the energy gap between the HOMO and LUMO in the hostmaterial, a direct-recombination light emission mechanism, i.e.,direction recombination of the carriers on the dopant when the carriersare trapped on the dopant, occurs as well as the above-described lightemission mechanism based on transfer of energy from the host to thedopant. This is the second light emission mechanism.

However, in a case where the dopant material and the host material arein such an energy level relationship, it is ordinarily difficult toseparately determine the contribution of each light emission mechanismto emission of light since transfer of energy is allowed according tothe conditions, and there is a possibility of both the light emissionmechanisms contributing to light emission.

A case where a triplet light emitting device is emitting light by theenergy transfer mechanism (first light emission mechanism) will bediscussed. In the conventional device structure, since the carrierrecombination region is the interface between the light emitting layerand the hole blocking layer, there is a possibility of transfer ofenergy to the hole blocking material as well as transfer of energy fromthe host material to the dopant material. However, since the absorptionspectrum of the hole blocking material is on an extremely shortwavelength side, there is, therefore, no overlap between the absorptionspectrum of the hole blocking layer and the light emission spectrum ofthe host material reported with respect to the conventional tripletlight emitting devices, and there is no possibility of transfer ofenergy between the host material and the hole blocking material. Thatis, the conventional triplet light emitting devices have such devicestructure that transfer of energy from the host material to the holeblocking material does not occur.

In contrast, in the device structure in accordance with the presentinvention, the carrier recombination region is the interface between thehole transporting layer containing a hole transporting material and theelectron transporting and light emitting layer containing a hostmaterial. In the device of the present invention, therefore, there is apossibility of transfer of energy from the host material to the holetransporting material. If energy transfer from the host material to thehole transfer material occurs, efficient emission of light cannot beachieved.

The relationship between the magnitudes of the energy gap value of thehost material and the energy gap value of the hole transporting materialcan be referred to as a rough guide with respect to energy transfer. Ifthe energy gap value of the host material is smaller than the energy gapvalue of the hole transporting material, it is difficult to excite thehole transporting material by transfer of energy from the host material.For this reason, it is preferred that the hole transporting materialhave an energy gap value larger than that of the host material in orderto avoid transfer of energy from the host material to the holetransporting material.

FIG. 3 is a relating energy diagram. The materials may be selected sothat the energy gap value A of the hole transporting material is largerthan the energy gap value B of the host material, as shown in FIG. 3.

A method of selecting, as a condition for prevention of energy transferbetween the host and hole transporting materials, a combination ofmaterials such that there is no overlap between the light emissionspectrum of the host material and the absorption spectrum of the holetransporting material may be used. When this method is used, it ispreferred that the absorption spectrum of the hole transporting materialis placed on the shorter-wavelength side of the light emission spectrumof the electron transporting material.

FIGS. 4A and 4B illustrate this condition. The positional relationshipbetween the spectrums in a case where transfer of energy occurs betweenthe host material and the hole transporting material is indicated inFIG. 4A, and the positional relationship between the spectrums in a casewhere transfer of energy does not occur between the host material andthe hole transporting material is indicated in FIG. 4B. According to thepresent invention, the positional relationship of FIG. 4B is preferred.

It is important to consider a condition other than those described aboveif a host material is selected such that both the energy levels of theHOMO and LUMO of the dopant material are placed in the energy gapbetween the HOMO and LUMO of the host material, because in such a casethe direct-recombination light emission mechanism (second light emissioncondition) is taken into consideration.

In such a case, it is suitable to set the value of the ionizationpotential indicating the HOMO of the hole transporting material to alarger value in order to facilitate injection of the hole carrier fromthe hole transporting material to the dopant material. That is, acombination of materials is selected such that the ionization potentialof the hole transporting material is higher than that of the dopantmaterial. If the ionization potential of the hole transporting materialis excessively high, the facility with which holes are injected from theanode into the hole transporting material is reduced. In such a case, ahole injection layer may be provided between the anode and the holetransporting layer to facilitate injection.

It is thought that the dopant traps the electron carrier through theelectron-transporting host. In a case where electrons not trapped by thedopant reach the interface on the hole transporting layer by movingthrough the electron transporting layer, the electrons reaching theinterface enter the hole transporting layer if the difference betweenthe LUMO level of the hole transporting material and the LUMO level ofthe host material is small. In such a case, electrons are not confinedin the electron transporting layer and efficient recombination cannot beachieved. To avoid such a situation, it is desirable to set thedifference between the LUMO levels of the hole transporting material andthe electron transporting material which is a host material to a valuelarge enough to block electrons. Preferably, this difference is 0.2 eVor greater.

More concrete examples of a method of fabricating the triplet lightemitting device of the prevent invention and materials used in thedevice will next be described.

A device fabrication method of the present invention shown in FIG. 2 isperformed as described below. First, a hole transporting material isdeposited on a substrate with an anode (ITO). Next, an electrontransporting material (host material) and a triplet light emittingmaterial (dopant) are codeposited. Finally, a cathode is formed bydeposition. The dopant concentration at the time of codeposition of thehost material and the dopant material is adjusted to about 8 wt %.Finally, sealing is performed to complete the organic light emittingdevice.

Materials which can be suitably used as a hole injection material, ahole transporting material, an electron transporting material (hostmaterial), and a triplet light emitting material (dopant material) inthe device of the present invention are shown below. However, materialsusable in the device of the present invention are not limited to thoseshown below.

As the effective hole injecting material among organic compounds, thereis a porphyrin-based compound, phthalocyanine (hereinafter referred toas “H₂Pc”), copper phthalocyanine (hereinafter referred to as “CuPc”),or the like. In addition, a material which has a smaller ionizationpotential than the hole transporting material to be used and a holetransporting function can also be used as the hole injecting material.There is also used a material obtained by chemically doping a conductivepolymer compound, for example, polyaniline or polyethylenedioxythiophene (hereinafter referred to as “PEDOT”) doped with sodiumpolystyrene sulfonate (hereinafter referred to as “PSS”). Alternatively,a polymer compound as an insulator is effective in flattening the anode,so that polyimide (hereinafter referred to as “PI”) is widely used.Furthermore, there is also used such an inorganic compound as a metalthin film of gold, platinum, or the like or a microthin film of aluminumoxide (hereinafter referred to as “alumina”).

As the effective hole transporting material, there is a material havingan energy gap value larger than that of the electron transportingmaterial to be used as the host material. Also, it is preferable thatthe material has a larger ionization potential than the light emittingmaterial or the absolute value of LUMO thereof is smaller than that ofthe electron transporting material by 0.2 eV or more.

Examples of the hole transporting material having a large energy gapvalue which is preferable for the device of the present inventioninclude: 4,4′,4″-tris(N-carbazole)triphenylamine (hereinafter referredto as “TCIA”) represented by the following structural formula 1;1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene (hereinafter referredto as “o-MTDAB”) represented by the following structural formula 2;1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene (hereinafter referredto as “m-MTDAB”) represented by the following structural formula 3;1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene (hereinafter referredto as “p-MTDAB”) represented by the following structural formula 4; and4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane (hereinafterreferred to as “BPPM”) represented by the following structural formula5.

On the other hand, 4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl(hereinafter referred to as “TPD”) which is an aromatic amine-basedcompound and used most widely and α-NPD as its derivative have smallerenergy gap values than the compounds represented by the structuralformulae 1 to 5, and are therefore difficult to use for the device ofthe present invention. Table 1 shows the energy gap values (actuallymeasured values) of the compounds represented by the structural formulae1 to 5, α-NPD, and TPD.

TABLE 1 Material Energy gap [eV] TCTA 3.3 o-MTDAB 3.6 m-MTDAB 3.5p-MTDAB 3.6 BPPM 3.6 TPD 3.1 α-NPD 3.1

A stable material is preferred as an electron transporting material usedas a host. For example, a selection may be made from a number of metalcomplexes of high stability. Materials used as a host material must havean energy gap value larger than that of the triplet light emittingmaterial, which is a dopant. Different host materials are selectedaccording to the light emitting materials used. Examples of electrontransporting materials usable as a host are shown below.

According to the present invention, as an example of a material that canbe used as the host material with respect to a blue light emittingmaterial, there is a material in which light emission spectrum can beseen at an extremely short wavelength as of ultraviolet region, forexample, 2,2′,2″-(1,3,5-benzenetrile)tris-[α-phenyl-1H-benzimidazole](hereinafter referred to as “TPBI”) represented by the followingstructural formula 6.

According to the present invention, examples of the host material withrespect to the green light emitting material include:lithiumtetra(2-(2-hydroxyphenyl)benzooxazolatoboron (hereinafterreferred to as “LiB(PBO)₄”) represented by the following structuralformula 7;bis(2-(2-hydroxyphenyl)benzooxazolato)(triphenylsilanolato)aluminum(hereinafter referred to as “SAlo”) represented by the followingstructural formula 8;bis(2-(2-hydroxyphenyl)benzothiazolato)(triphenylsilanolato)aluminum(hereinafter referred to as “SAlt”) represented by the followingstructural formula 9; 2-(2-hydroxyphenyl)benzooxazolatolithium(hereinafter referred to as “Li(PBO)”) represented by the followingstructural formula 10; and(2-(2-hydroxyphenyl)benzooxazolato)-diphenylboron (hereinafter referredto as “B(PBO)Ph₂”) represented by the following structural formula 11.In addition to these, it is possible to use as the host material thematerial that can emit blue light.

According to the present invention, examples of the host material withrespect to the red light emitting material include: Alq represented bythe following structural formula 12;bis(2-methyl-8-quinolinolato)(triphenylsilanolato)aluminum (hereinafterreferred to as “SAlq”) represented by the following structural formula13; bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(hereinafter referred to as “BAlq”) represented by the followingstructural formula 14; lithiumtetra(2-methyl-8-hydroxy-quinolinato)boron(hereinafter referred to as “LiB(mq)₄”) represented by the followingstructural formula 15; (2-methyl-8-quinolinolato)-diphenylboron(hereinafter referred to as “BmqPh”) represented by the followingstructural formula 16; andbis(2-methyl-8-quinolinolato)aluminiumhydroxide (hereinafter referred toas “Almq₂(OH)”) represented by the following structural formula 17. Inaddition to these, it is possible to use as the host material thematerial that can emit blue light or the material that can emit greenlight.

Note that the energy gap values (actually measured values) in accordancewith some of the host materials described above are shown in Table 2.

TABLE 2 Material Energy gap [eV] TPBI 3.5 LiB(PBO)₄ 3.1 SAlo 3.2 SAlt3.0 Alq 2.7 SAlq 3.0 LiB(mq)₄ 3.0

Examples of the triplet light emitting material as a dopant mostlyinclude complexes having a central metal of iridium or platinum.However, any material may be adopted as long as it emits phosphorescenceat a room-temperature. As such a material, for example, there are PtOEP,Ir(ppy)₃, bis(2-phenylpyridinato-N,C^(2′))acetylacetonatoiridium(hereinafter referred to as “acacIr(ppy)₂”),bis(2-(4′-trile)-pyridinato-N,C^(2′))acetylacetonatoiridium (hereinafterreferred to as “acacIr(tpy)₂”), andbis(2-(2′-benzothienyl)pyridinato-N,C^(3′))acetylacetonatoiridium(hereinafter referred to as “acacIr(btp)₂”).

Note that the energy gap values (actually measured values) in accordancewith the dopant materials described above are shown in Table 3.

TABLE 3 Material Energy gap [eV] Ir(ppy)₃ 2.4 acacIr(ppy)₂ 2.4acacIr(tpy)₂ 2.4 acacIr(btp)₂ 2.3

As the electron injecting material, the electron transporting materialdescribed above can be used. However, such an electron transportingmaterial (BCP, OXD7, or the like) that is used as the hole blockingmaterial is low in stability, and thus it is inappropriate as theelectron injecting material. In addition, there is often used amicrothin film made of an insulator, for example, alkali metal halidesuch as lithium fluoride or alkali metal oxide such as lithium oxide.Also, an alkali metal complex such as lithium acetylacetonate(hereinafter referred to as “Li(acac)”) or 8-quinolinolato-lithium(hereinafter referred to as “Liq”) is effective.

A combination of materials is selected from the above-describedmaterials having the desired functions to be used in the organic lightemitting device of the present invention. Thus, a high-efficiencyorganic light emitting device which can be fabricated by a simplerprocess in comparison with the conventional triplet light emittingdevices, which has improved stability, and which has a light emissionefficiency substantially equal to that of the conventional triplet lightemitting devices can be provided.

Embodiments of the organic light emitting device of the presentinvention shown in FIG. 2 will be described below.

Embodiment 1

First, a 40 nm-thick layer of BPPM, which is a hole transportingmaterial, is deposited on glass substrate 101 with ITO film formed asanode 102 and having a thickness of about 100 nm. Hole transportinglayer 103 is thereby formed.

After fabrication of the hole transporting layer, acacIr(tpy)₂, which isa triplet light emitting material, and TPBI, which is an electrontransporting material (host material), are codeposited in proportions ofabout 2:23 (weight ratio). That is, acacIr(tpy)₂ is dispersed at aconcentration of about 8 wt % in TPBI. A 50 nm-thick codeposited film isthereby formed. This film is electron transporting and light emittinglayer 104.

Finally, Mg and Ag are codeposited at an atomic ratio of 10:1 to formcathode 105. A 150 nm-thick film for cathode 205 is thereby formed. Atriplet light emitting device which emits green light derived fromacacIr(tpy)₂ is thus obtained.

FIGS. 5A to 5D are graphs showing an initial characteristic and a lightemission spectrum in this device. Even though the simple two-layerstructure was formed, a device characteristic of high efficiency, i.e.,a maximum external quantum efficiency of about 10%, was exhibited.

Embodiment 2

A device in accordance with the present invention was fabricated byusing a hole transporting material (satisfying the condition inaccordance with the present invention) different from that in Embodiment1.

First, a 40 nm-thick layer of o-MTDAB, which is a hole transportingmaterial, is deposited on glass substrate 101 with ITO film formed asanode 102 and having a thickness of about 100 nm. Hole transportinglayer 103 is thereby formed.

After fabrication of the hole transporting layer, acacIr(tpy)₂, which isa triplet light emitting material, and TPBI, which is an electrontransporting material (host material), are codeposited in proportions ofabout 2:23 (weight ratio). That is, acacIr(tpy)₂ is dispersed at aconcentration of about 8 wt % in TPBI. A 50 nm-thick codeposited film isthereby formed. This film is electron transporting and light emittinglayer 104.

Finally, Mg and Ag are codeposited at an atomic ratio of 10:1 to formcathode 205. A 150 nm-thick film for cathode 205 is thereby formed. Atriplet light emitting device which emits green light derived fromacacIr(tpy)₂ is thus obtained.

FIGS. 6A to 6D are graphs showing an initial characteristic and a lightemission spectrum of this device. A high-efficiency device can befabricated as in Embodiment 1.

Embodiment 3

An organic light emitting device in accordance with the presentinvention was fabricated by using as a host material a hole transportingmaterial (satisfying the condition in accordance with the presentinvention) different from that in Embodiment 1. The fabrication methodis the same as that in Embodiments 1 and 2. BPPM is used as a holetransporting material, SAlt is used as a host, i.e., the electrontransporting material, and acacIr(tpy)₂ is used as a dopant. A tripletlight emitting device which emits green light derived from acacIr(tpy)₂can be obtained.

FIGS. 7A to 7D show an initial characteristic and a light emissionspectrum of this device. A high-efficiency device having a lightemission efficiency substantially equal to that in the conventionaltriplet light emitting devices can be fabricated as in Embodiment 1 or2.

Embodiment 4

By using a triplet light emitting material different from Embodiment 1,2, or 3 as a dopant, an organic light emitting device having a lightemission color different from that of Embodiment 1, 2, or 3 is prepared.The method for preparation is the same as that of Embodiments 1, 2, and3. BPPM is used as the hole transporting material, TPBI is used as theelectron transporting material, andbis(2-(2′,4′-difluorophenyl)pyridinato-N,C2′)picolatoiridium is used asthe dopant. It is possible to obtain the triplet light emitting devicewhich emits blue light derived from the dopant material.

FIGS. 8A to 8D show an initial characteristic and a light emissionspectrum of this device. A high-efficiency device having a lightemission efficiency substantially equal to, that in the conventionaltriplet light emitting devices can be fabricated as in Embodiment 1, 2,or 3.

Comparative Example 1

A device of a structure similar to that of the conventional tripletlight emitting device shown in FIG. 12 was manufactured and itscharacteristics were compared with those of the devices of the presentinvention.

First, a 40 nm-thick layer of α-NPD, which is a hole transportingmaterial, is deposited on glass substrate 1101 with ITO film formed asanode 1102 and having a thickness of about 100 nm. Hole transportinglayer 1103 is thereby formed.

After fabrication of the hole transporting layer, acacIr(tpy)₂, which isa triplet light emitting material, and CBP, which is a host material,are codeposited in proportions of about 2:23 (weight ratio). That is,acacIr(tpy)₂ is dispersed at a concentration of about 8 wt % in CBP. A50 nm-thick codeposited film is thereby formed. This film is lightemitting layer 1104.

After the formation of the light emitting layer, a 20 nm-thick film ofBCP, which is a hole blocking material, is deposited to form holeblocking layer 1105. A 30 nm-thick film of Alq, which is an electrontransporting material is deposited to form electron transporting layer1106.

Finally, Mg and Ag are codeposited at an atomic ratio of 10:1 to formcathode 1107. A 150 nm-thick film for cathode 1107 is thereby formed. Atriplet light emitting device which emits green light derived fromacacIr(tpy)₂ is thus obtained.

FIGS. 9A to 9D show an initial characteristic and a light emissionspectrum of this device. From comparison between this comparativeexample and each of Embodiments 1, 2, and 3, it can be understood thatthe device of the present invention in each Embodiment has the same highefficiency as the conventional device. It was confirmed thatsufficiently high device characteristics were exhibited even though nohole blocking layer was used.

Comparative Example 2

In this comparative example, characteristics of a triplet light emittingdevice of a two-layer structure in which a hole transporting materialnot satisfying the device conditions in accordance with the presentinvention is used are examined.

The same fabrication method as that in the Embodiments of the presentinvention is used. However, a combination of a hole transportingmaterial and a host material is used such that the energy gap value ofthe hole transporting material used is smaller than that of the hostmaterial. TPD is used as the hole transporting material, TPBI is used asthe host material, which is an electron transporting material, andacacIr(tpy)₂ is used as a dopant.

FIGS. 10A to 10D show an initial characteristic and a light emissionspectrum of this device. The device using TPD as a hole transportingmaterial has a considerably low light emission efficiency for a tripletlight emitting device. A spectral component (about 400 nm) correspondingto emission from TPD other than emission from acacIr(tpy)₂ is observed,as seen in the light emission spectrum. A reduction in efficiencyresults from this. Thus, the initial characteristic of the device isinferior if a material not satisfying the condition is used.

Comparative Example 3

In this comparative example, characteristics of a triplet light emittingdevice of a two-layer structure in which a hole transporting materialnot satisfying the device conditions in accordance with the presentinvention is used as in Comparative Example 2 are examined.

The same fabrication method as that in the Embodiments of the presentinvention is used. However, a combination of a hole transportingmaterial and a host material is used such that the energy gap value ofthe hole transporting material used is smaller than that of the hostmaterial. In this example, α-NPD is used as the hole transportingmaterial, TPBI is used as the host material, which is an electrontransporting material, and acacIr(tpy)₂ is used as a dopant.

FIGS. 11A to 11D show an initial characteristic and a light emissionspectrum of this device. The device using α-NPD as a hole transportingmaterial has a considerably low light emission efficiency for a tripletlight emitting device, as in Comparative Example 2. A spectralcomponent, (about 440 nm) corresponding to emission from α-NPD which isa hole transporting material is observed, as in Comparative Example 2. Areduction in efficiency results from this. Thus, the initialcharacteristic of the device is inferior if a material not satisfyingthe condition is used.

If the present invention is carried out, a triplet light emitting devicehaving a light emission efficiency substantially equal to that of theconventional triplet light emitting devices can be obtained in a simpledevice structure. Also, the layer in which an unstable material is usedis removed to enable a stable organic light emitting device to beprovided.

1. A light emitting device comprising an anode, an organic compoundfilm, and a cathode, the organic compound film comprising: a holetransporting layer comprising a hole transporting material; and anelectron transporting layer in contact with the hole transporting layerand comprising an electron transporting material, wherein a complexincluding iridium is added in the electron transporting layer, andwherein the electron transporting material comprises a complex including2-methyl-8-quinoline unit.
 2. A light emitting device according to claim1, further comprising a hole injection layer in contact with the anode.3. A light emitting device according to claim 1, further comprising anelectron injection layer in contact with the cathode.
 4. A lightemitting device according to claim 1, wherein an energy differencebetween a highest occupied molecular orbit level and a lowest unoccupiedmolecular orbit level in the hole transporting material is larger thanthat between a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level in the electron transporting material.5. A light emitting device according to claim 1, wherein an absorptionspectrum of the hole transporting material and a light emission spectrumof the electron transporting material do not overlap each other.
 6. Alight-emitting device according to claim 1, wherein an absorptionspectrum of the hole transporting material is on a shorter-wavelengthside of a light emission spectrum of the electron transporting material.7. A light emitting device comprising an anode, an organic compoundfilm, and a cathode, the organic compound film comprising: a holetransporting layer comprising a hole transporting material; and anelectron transporting layer in contact with the hole transporting layerand comprising an electron transporting material, wherein a complexincluding iridium is added in the electron transporting layer, andwherein the electron transporting material comprises a complex includingbenzoxazole unit.
 8. A light emitting device according to claim 7,further comprising a hole injection layer in contact with the anode. 9.A light emitting device according to claim 7, further comprising aelectron injection layer in contact with the cathode.
 10. A lightemitting device according to claim 7, wherein an energy differencebetween a highest occupied molecular orbit level and a lowest unoccupiedmolecular orbit level in the hole transporting material is larger thanthat between a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level in the electron transporting material.11. A light emitting device according to claim 7, wherein an absorptionspectrum of the hole transporting material and a light emission spectrumof the electron transporting material do not overlap each other.
 12. Alight-emitting device according to claim 7, wherein an absorptionspectrum of the hole transporting material is on a shorter-wavelengthside of a light emission spectrum of the electron transporting material.13. A light emitting device comprising an anode, an organic compoundfilm, and a cathode, the organic compound film comprising: a holetransporting layer comprising a hole transporting material; and anelectron transporting layer in contact with the hole transporting layerand comprising an electron transporting material, wherein a complexincluding iridium is added in the electron transporting layer, andwherein the electron transporting material comprises a complex includingbenzothiazole unit.
 14. A light emitting device according to claim 13,further comprising a hole injection layer in contact with the anode. 15.A light emitting device according to claim 13, further comprising aelectron injection layer in contact with the cathode.
 16. A lightemitting device according to claim 13, wherein an energy differencebetween a highest occupied molecular orbit level and a lowest unoccupiedmolecular orbit level in the hole transporting material is larger thanthat between a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level in the electron transporting material.17. A light emitting device according to claim 13, wherein an absorptionspectrum of the hole transporting material and a light emission spectrumof the electron transporting material do not overlap each other.
 18. Alight-emitting device according to claim 13, wherein an absorptionspectrum of the hole transporting material is on a shorter-wavelengthside of a light emission spectrum of the electron transporting material.19. A light emitting device comprising an anode, an organic compoundfilm, and a cathode, the organic compound film comprising: a holetransporting layer comprising a hole transporting material; and anelectron transporting layer in contact with the hole transporting layerand comprising an electron transporting material, wherein a complexincluding iridium is added in the electron transporting layer, andwherein the electron transporting material comprises a complex includingbenzimidazole unit.
 20. A light emitting device according to claim 19,further comprising a hole injection layer in contact with the anode. 21.A light emitting device according to claim 19, further comprising aelectron injection layer in contact with the cathode.
 22. A lightemitting device according to claim 19, wherein an energy differencebetween a highest occupied molecular orbit level and a lowest unoccupiedmolecular orbit level in the hole transporting material is larger thanthat between a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level in the electron transporting material.23. A light emitting device according to claim 19, wherein an absorptionspectrum of the hole transporting material and a light emission spectrumof the electron transporting material do not overlap each other.
 24. Alight-emitting device according to claim 19, wherein an absorptionspectrum of the hole transporting material is on a shorter-wavelengthside of a light emission spectrum of the electron transporting material.